Here's an idea: make it usable outdoors.
The AI Tornado works well indoors, but rather poorly outdoors. In fact, due to the poor ourdoor performance of most grenades (be it commercial, home made/pyro-based, or the Hokatsu Thunder grenades [which don't project BBs]) most games [here anyway] now rely on a kill radius rather than actually BB hits.
That means having a grenade which can easily be seen and heard going off would be more useful than one that attempts to scatter BBs (the only ones I've seen which did this well were pyro-based and used charges which were far too powerful to be used safely in games, i.e.: cherrybombs etc)
Another problematic aspect of using reusable grenades outdoors is having to retrieve the grenade: obviously its not always possible to do so immediately after throwing it/in the middle of a firefight. Add to this low light conditions, dense vegetation and uneven terrain and even if you know exactly where the grenade landed it can be difficult to find. In this respect the addition of "locating" feature (beacon?) which would activate after the grenade has gone off would be useful; something like an intermittent strobe and loud chirp (say 0.5s every 15 seconds so it isn't too annoying but remains useful).
Obviously the beacon needs to have power to operate for several hours and the strobe should be able to survive the impacts from being thrown over and over.
And as with most grenades, it should have some kind of soft/rubberized exterior so that it doesn't injure anyone if it lands on someone's head or smacks someone in the face (e.g.; throw a grenade through a window just as someone pops up behind it).